GaN compound semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), etc. are attracting much attention as materials for blue-ray light-emitting diodes (LEDs) and laser diodes (LDs). Particularly because GaN compound semiconductors have good heat resistance and environmental resistance, development has been conducted to apply them to electronic devices to utilize their characteristics.
GaN-growing substrates widely used at present are sapphire base substrate. The production of GaN may be carried out generally by epitaxially growing GaN on a single-crystal sapphire base substrate by a metal-organic vapor phase epitaxy (MOVPE) method, etc.
However, because the sapphire base substrate has a different lattice constant from that of GaN, the direct growth of GaN on the sapphire base substrate fails to provide a single-crystal GaN layer. Thus, JP A 4-297023 discloses a method of growing a buffer layer of AlN or GaN on a sapphire base substrate at a low temperature, such that this low-temperature-grown buffer layer relaxes lattice strain, and growing GaN thereon. The use of a low-temperature-grown nitride layer as a buffer layer enables the epitaxial growth of single-crystal GaN. However, even this method cannot remove the discrepancy of a lattice between a substrate and a crystal, resulting in GaN having numerous defects, which are expected to cause problems in the production of GaN laser diodes (LDs).
For the above reasons, the development of self-supported GaN substrates is desired. Because it is difficult to grow a large GaN ingot from a melt like Si and GaAs, various attempts are conducted to grow GaN, by an ultra-high temperature, high-pressure method (S. Porowski and I. Grzegoty, “J. Crystal Growth,” Vol. 178, p. 174, 1997), a flux method (H. Yamanera, etc., “Chem. Mater.,” Vol. 9, p. 413, 1997), a hydride vapor-phase epitaxy (HVPE) method (H. P. Maruska and J. J. Tietjen, “Appl. Phys. Lett.,” Vol. 15, p. 327, 1969), etc.
However, the above methods fail to provide high-quality, large GaN single crystals usable for practical applications. For instance, because the ultra-high-temperature, high-pressure method needs pressures at several tens of thousands of atmospheres and temperatures at several thousands of degrees centigrade as growth conditions, it is difficult to produce large crystals. Accordingly, only crystals of about several millimeters in diameter and about several tens of microns in thickness can be obtained at present.
Though the flux method can conduct crystal growth under the conditions of several hundreds of atmospheres and about 1000° C., it can produce only as small crystals as several millimeters in diameter and several tens of microns in thickness. The flux method suffers from problems such as nitrogen dissociation, the inclusion of fluxes of Na, Ca, etc. into crystals. Also, because the control of the generation of nuclei is difficult at an early stage of growth, polycrystals are likely to be included in single crystals produced.
The HVPE method has succeeded in producing crystals of about 2 inches in diameter. However, because of growth conditions causing a vigorous vapor phase reaction, foreign matters such as polycrystals, etc. are easily included in crystals generated, resulting in poor crystallinity. In addition, nitrogen dissociation occurs remarkably depending on the growth conditions, so that inherently transparent crystals are likely to be colored.
Because these defects occurring in the crystals cause problems in the production of devices, they should be removed as much as possible. JP A 2003-178984 discloses a method of forming a GaN layer and a titanium layer on a sapphire base substrate, heat-treating the substrate in an atmosphere containing a hydrogen gas or a hydrogen-containing compound to generate voids in the GaN layer, and forming a further GaN layer. In this method, a defect density is reduced by using a half width of an X-ray diffraction rocking curve as an index of crystallinity. However, because known evaluation means such as a transmission electron microscope (TEM), an X-ray diffraction (XRD), etc. evaluate the crystallinity of crystals extremely locally, it is difficult to evaluate the entire surfaces of large-area crystals obtained by actual crystal growth processes. Therefore, it has so far been difficult to improve the characteristics of crystal substrates.